The present invention relates to a wellhead assembly for accessing an annulus within a well, especially a subsea well, in particular an annulus between adjacent casing strings within the well. The present invention further relates to a method of accessing an annulus within a well, in particular a subsea well, and the use of the method in the injection of fluids into the well or the production of fluids from the well.
A range of well operations requires access to an annulus between adjacent casing strings within the well. For example, it is often required to produce or displace fluids from the well through such an annulus, such as when drilling operations are underway. In addition, it is also a known practice to inject fluids into the well and underground formations into which the well extends through such an annulus. One operation involving such an injection of fluid is the disposal of cuttings.
The drilling of an oil or gas well, for example a subsea well, results in the formation of small fragments of rock and other matter, known as cuttings, from the various formations through which the-well is drilled. The cuttings are removed from the well as they are formed by the drill bit by being entrained in a drilling mud pumped down the well and returned to the surface vessel or platform. The cuttings are typically recovered from the drilling mud by a separation process and the mud reused in the well operations. In the past, at offshore locations, it has been common practice to dispose of the cuttings separated from the drilling mud in this way by dumping them in the sea. This practice has proven acceptable in the past, as the environmental impact from the negligible amounts of entrained oil based mud in the cuttings was low. Additionally, may companies have changed their practice to use synthetic drilling mud that is environmentally friendly.
Recently, however, it has become favored to employ oil based drilling muds, as such mud formulations offer a number of advantages. For example, oil based muds improve the stability of the well bore, improve the performance of the drill bit by providing better lubrication and removal of cuttings as they are formed, and reduce the torque generated in the drill string during use. For these reasons, oil based drilling muds have been finding increasing use. While offering advantages during the drilling operation, the oil based mud formulations present a problem with respect to disposal. Cuttings separated from the oil based muds after recovery from the well are inevitably contaminated with the oil based formulation. Washing the cuttings has been attempted, but only removes the mud from the surface of the cuttings particles, leaving oil in the cracks and pores of the fragments. It is no longer possible to dispose of cuttings recovered from an offshore well using oil based drilling muds in the same manner as when water based muds are employed by simply pouring the cuttings into the sea, as the cuttings give rise to an adverse environmental impact. The corresponding environmental regulations have become more stringent year by year, decreasing the allowable quantities of entrained oil based mud present in the washed cuttings.
Accordingly, it has been the practice to dispose of the cuttings by injecting them into a well and into subsurface formations. To facilitate this, it has been the practice to grind the cuttings and suspend them in a suitable liquid to form a pumpable slurry, which may then be injected into a subsurface formation through an annulus between adjacent casings in the well. This has been common practice in environmentally sensitive areas, such as the north slope of Alaska, for many years.
US Pat. No. 4,942,929 discloses a method for the disposal and reclamation of drilling wastes, in which construction grade gravel is separated from drilling cuttings produced during well drilling operations. The solids that are not so recovered are formed as a slurry with the remaining clays, silts and spent drilling fluid and conducted to a second well, remote from the well being drilled, into which the slurry is injected. Centrifugal pumps or mechanical agitators are used to disperse the fine solids in the slurry to assist in the injection process.
A drill cuttings disposal method and system are disclosed in U.S. Pat. No. 5,129,469. In the method and system disclosed, drill cuttings produced during well drilling operations are brought to the surface and separated from the drilling mud, mixed with a suitable liquid, such as sea water and the mixture subjected to grinding to form a slurry. The slurry may then be pumped into a selected zone of the well for disposal.
U.S. Pat. No. 5,341,882 discloses a method for the disposal of well drilling cuttings, in which the cuttings are solidified by combining the cuttings with water and blast furnace slag. The resulting mixture is injected into the annulus between two wellbore casings, where it solidifies to form a cement.
U.S. Pat. No. 5,884,715 discloses a method and apparatus for injecting cuttings into a well while drilling operations are in progress. Two embodiments are discussed in the disclosure. The first method requires a predrilled well bore to be bored adjacent to and extending away from the well being drilled. The predrilled well bore is used as a depository for the drilling cuttings produced from the well being drilled. The second embodiment requires an injection tube to be installed within the well being drilled alongside the casings set into the well, through which access can be gained to subsurface formations into which the cuttings may be injected. A further embodiment employs an annulus between adjacent casings in the well in order to gain access to underground formations. It is noted that the embodiments disclosed in U.S. Pat. No. 5,884,715 relate to the injection of cuttings into a well having a wellhead accessible on land. While subsea operations are mentioned, little information is given regarding the injection of cuttings into subsea wells.
A subsea wellhead typically comprises a conductor pipe extending below the sea bed in the well, the upper portion of which extends from the well and forms a conductor housing. A high pressure housing is landed in the conductor housing, on which is typically mounted a blowout preventer (BOP) stack by means of a BOP guide funnel. Successively smaller casings are landed in the wellhead, suspended from casing hangers secured within the conductor pipe or the high pressure housing. The pressure boundary region of the wellhead assembly is that region of the assembly between the first or upper end of the high pressure housing and the lowermost casing hanger seal assembly. A guide base is often employed, which comprises a structure extending around the wellhead and mounted to the conductor housing.
A subsea well injection system is disclosed in U.S. Pat. No. 5,085,277, for injecting unwanted slurries and other fluids arising from drilling or other downhole operations into a subsea well. The slurry or other fluid is injected through a drilling guide base positioned around the well on an underwater surface. The system employs a dedicated guide base, which comprises pipework on the guide base leading to a port in the conductor casing of the well, thus gaining access to the annulus between the conductor casing and the adjacent inner casing. A fail safe isolation valve is provided on the guide base and joined to the pipework. A coupling is provided to connect the isolation valve to a surface vessel or platform. The wellhead is modified to provide a port in the housing, in order to gain access to an annulus between casings within the well. With a single port in the outermost casing of the well, fluids may be injected into the outermost annulus of the well. If access is required to an inner annulus, similar ports are required in the casings disposed radially outwards of the inner annulus to provide a flow path to the pipework extending from the guide base.
In U.S. Pat. No. 5,339,912, there is disclosed a cuttings disposal system in which an injection adapter is employed to allow a slurry of cuttings to be injected into a well. The well, designated an xe2x80x9cinjection wellxe2x80x9d, has an inner and an outer wellhead housing with at least one casing hanger and a respective inner casing installed in the inner wellhead housing. The casing hanger is formed with a port through it, connecting the bore of the well with the annulus between the inner casing and the outer casing of the well. When it is desired to inject cuttings into the well, an injection adapter is landed in the wellhead so as to extend into the bore of the well, allowing a central bore in the injection adapter to connect, through a port in the side of the injection adapter body, with the port in the casing hanger. The central bore in the injection adapter is connected by pipework to a pump at the surface, by means of which a slurry of cuttings may be injected through the injection adapter and into the annulus in the well. It is noted that, with the injection adapter landed in the well, access to the well for conducting other operations is denied, until the cuttings injection operation is ceased and the injection adapter removed.
U.S. Pat. No. 5,255,745 describes a method and apparatus for providing a remotely operable connection to establish access to an annulus within a wellhead assembly. The apparatus requires a port in the wellhead assembly. A valve is positioned to seal with the port by remote means using a ramp assembly supported on a guide base positioned around the wellhead. U.S. Pat. No 5,255,745 does not, however, address the wellhead equipment necessary in order to gain access in the wellhead to the annulus mentioned.
A cuttings injection wellhead system for use in subsea wells is disclosed in U.S. Pat. No. 5,662,169. The wellhead system employs a wellhead having a conductor casing, to which is mounted a conductor housing and around which a guide base is provided. A high pressure housing is landed in the conductor housing. The wellhead system comprises an extension to the conductor housing extending between the lower end of the conductor housing and the conductor casing. A port is formed in the conductor housing extension below the guide base, allowing access to the interior of the conductor housing. A similar extension is provided on the lower end of the high pressure housing, formed with a corresponding port aligned with the port in the conductor casing. An inner casing is suspended from a casing hanger disposed within the high pressure housing. The ports in the extensions to the conductor housing and high pressure housing provide access to the annulus around the inner casing, into which a slurry of drilling cuttings may be injected. The pipework necessary to connect with the port in the conductor housing extension depends from the guide base provided around the wellhead assembly. The wellhead system of U.S. Pat. No. 5,662,169 requires the use of a modified conductor housing and high pressure housing, both of which must be provided with extensions through which aligned ports must be bored. In addition, the system of U.S. Pat. No. 5,662,169 requires the use of a dedicated guide base with the necessary pipework and connections in order to allow cuttings injection to proceed.
In a paper entitled xe2x80x9cSubsea Cuttings Injection Guide Base Trialxe2x80x9d presented at the Offshore European Conference, Sep. 7 to 10, 1993, Ferguson et al. disclosed the results of field trials conducted to test a permanent guide base and wellhead assembly modified to allow cuttings injection. A modified permanent guide base was employed having a pipe connecting through the guide base to a port in an extension welded to the conductor housing of the wellhead. A similar extension was provided on the lower end of the high pressure housing, through which a port was formed to align with the port in the extension to the conductor housing and provide access to an inner annulus of the wellhead assembly. As with the system of U.S. pat. No. 5,662,169, a dedicated guide base is required in this system in order to provide the possibility of cuttings injection, together with modifications to several of the wellhead components.
A similar cuttings injection system is disclosed by Saasen et al. in a paper entitled xe2x80x9cThe First Cuttings Injection Operation Worldwide in a Subsea Annulus: Equipment and Operational Experiencexe2x80x9d, presented at the SPE Annual Technical Conference and Exhibition, Sep. 27 to 30, 1998. Again, this system employs a modified guide base, required to be larger than conventional guide bases, through which access is gained to a port formed in the conductor housing. A similar port is provided in the high pressure housing, aligned with the port in the conductor housing, in order to access an annulus between the high pressure housing, and its associated casing, and a casing suspended from a casing hanger secured in the bore of the high pressure housing. Again, the system of Saasen et al. requires a modified, dedicated guide base to be provided in order to inject cuttings into an annulus within the wellhead assembly. Further, in the system of Saasen et al. seal cartridges are required to be provided within the conductor housing around the high pressure housing both above and below the ports in the conductor housing and high pressure housing, in order to avoid ingress of the cuttings slurry into the annulus between the conductor housing and the high pressure housing.
It is noted that the prior art teaches, in general, that it is required to employ a guide base in order to gain access to an annulus within the wellhead assembly and the well, in order to effect cuttings injection into a subsea wellhead. Reference is made to U.S. Pat. No. 5,085,077, U.S. Pat. No. 5,255,745, U.S. Pat. No. 5,662,169 and the papers by Ferguson et al. and Saasen et al. In addition, the systems proposed required significant modifications to the components of the wellhead assembly in order to provide access to the annulus of choice within the well. It would be advantageous if a method and apparatus could be found for accessing an annulus within a wellhead assembly and the well without requiring substantial modifications to the conventional wellhead assembly components. It would be a further advantage if the need for a guide base, in particular a modified, dedicated guide base, could be dispensed.
It is also noted that, in general, the aforementioned proposals require an access port to be formed in the conductor housing of the wellhead assembly. It would also be an advantage if the need to provide an opening or port in the conductor housing could be removed and access provided to an annulus within the wellhead assembly and the well without requiring modification to the conductor housing or the conductor casing.
As can also be seen from a review of the prior art, a number of the proposals to date require a port to be formed in the wall of a casing hanger within the wellhead assembly, in order to access an annulus within the well. Reference is made to U.S. Pat. No. 5,339,912. This design requires that the port in the casing hanger be sealed, when it is required to conduct well operations through the bore of the wellhead assembly that do not require access to the annulus. If the seals fail, the fluids in the well bore, being at a higher pressure than the annulus accessed by the port, would flow into the annulus. These fluids could be at a pressure in excess of the burst pressure of the outer casing, leading to a rupture of the casing. Alternatively, the fluids entering the annulus could find their way into a low pressure formation, from where they could access the surface, for example through a fault. This would in turn lead to harm to the subsea environment in the region of the breach. Finally, as has already been noted, access to the port in the casing hanger can only be obtained from within the bore of the wellhead assembly, requiring the use of a tool, which in turn closes the bore and prevent its use for other well operations.
There is clearly a need for a way to access an annulus within a wellhead assembly and the associated well, for example to inject drilling cuttings into the well, while keeping the modifications required to the conventional or existing equipment to a minimum. Further, it would be most advantageous to be able to have such access and carry out the required operations without requiring a guide base to be present. It would be of further advantage if access could be obtained to the annulus, for example to inject cuttings into the well, while drilling and other well operations were proceeding at the same time.
According to a first aspect of the present invention there is provided a wellhead assembly comprising:
a central bore through the wellhead assembly;
a pressure boundary region;
a port in the wellhead assembly having an opening in the central bore in the pressure boundary region of the wellhead assembly;
a first casing string secured at a first end within the wellhead assembly below the opening of the port in the central bore of the wellhead assembly; and
a second casing string secured at a first end within the wellhead assembly above the opening of the port in the central bore of the wellhead assembly;
thereby forming a fluid flowpath comprising the port in the wellhead assembly and an annulus between the first and second casing strings.
The wellhead assembly provides an arrangement for accessing an annulus within the wellhead assembly and the well requiring only minimal modification of the existing or conventional wellhead assembly components. In particular, it is possible to employ conventional casing strings and casing hangers, with no modification being required. In addition, the access to the annulus is provided without requiring extra seals to be employed to maintain the integrity of the flowpath and isolate it from the central bore of the wellhead assembly and the well. Indeed, the wellhead assembly simply requires the conventional subsea seals and pack-offs to secure and seal the casing strings and their associated casing hangers within the wellhead assembly.
It is also to be noted that, unlike the prior art, access is gained to an inner annulus within the wellhead assembly and the well without requiring the conductor housing to be penetrated. In this way, the integrity of the conductor housing is not compromised.
The wellhead assembly preferably comprises a high pressure housing, the port in the wellhead assembly being in the high pressure housing. In such an arrangement, the first casing string may be secured in the high pressure housing, for example using a conventional casing hanger, below the opening of the port. The second casing string may be secured in the high pressure housing above the opening of the port, again using a conventional casing hanger.
It is also to be understood that further casing hangers may be disposed either above and/or below the opening of the port into the central bore, from which further casing strings may be suspended. In the case in which further casing hangers and their respective casings are provided below the opening of the port in the central bore, access to two or more annuli between such casings may be provided, for example by providing the appropriate casing hanger with ports therethrough.
In a preferred embodiment, a sleeve is disposed within the wellhead assembly between the first ends of the first and second casing strings. The sleeve is formed with one or more ports through it, in order to connect the port in the wellhead assembly with the annulus between the first and second casing strings. The cross-sectional area of the port or ports in the sleeve may be greater than that of the port in the wellhead assembly. In this way, the sleeve acts to reduce the velocity of fluid passing through the flowpath. This is of particular importance when injecting a slurry of cuttings into the wellhead assembly and the well. By reducing the velocity of the particles in the slurry, the erosion of the casing hangers and casings by the slurry is reduced. The sleeve may also act as a wear sleeve, to further prevent erosion of and damage to the casing. To achieve this, the port in the sleeve is positioned relative to the port in the wellhead assembly such that a fluid entering or leaving the wellhead assembly is deviated in its path, causing the fluid to first impinge on the sleeve, before passing through the port in the sleeve. In a preferred arrangement, the port in the sleeve is disposed such that the flowpath between the port in the wellhead assembly and the annulus comprises two such deviations. It should be noted that standard hanger body design incorporates ports for the bypass of annulus fluid during casing hanger running and casing cementing operations. These ports may be used in addition to or in place of the ports in the sleeve through which the fluid passes.
The sleeve is preferably separate from the first and second casing strings and their hangers. In a preferred arrangement, the sleeve is arranged to be landed in the wellhead assembly together with the second casing string and its associated hanger.
In order to carry out operations in which fluid is passed into and out of the annulus within the well through the wellhead housing of the present invention, it is preferred to provide a riser interface or flowline assembly connected to the port in the wellhead assembly, by which fluid may be transported between a surface vessel or platform and the wellhead assembly using a riser. The interface assembly preferably includes at least one valve for regulating the flow of fluid along the flowpath, the valve preferably being a fail-safe closed valve. The interface assembly can be constructed and installed as a separate, independent assembly, without requiring the presence of a guide base, whether of conventional design or modified in any way.
In a further aspect, the present invention provides a method for providing access to an annulus in a well, the well having a wellhead assembly comprising a central bore therethrough, the annulus being formed between a first casing string and a second casing secured within the wellhead assembly and extending into the well, the method comprising:
providing a port in the wellhead assembly having an opening into the pressure boundary region of the central bore;
securing the first casing string at a first end in the wellhead assembly below the opening of the port in the central bore; and
securing the second casing string at a first end in the wellhead assembly above the opening of the port in the central bore;
thereby forming a fluid flowpath comprising the port in the wellhead assembly and the annulus between the first and second casing strings.
The method for providing access to an annulus in the well may employ a wellhead assembly having the features described above.
In a further aspect, the present invention provides a method for passing a fluid through an annulus in a well, the well having a wellhead assembly comprising a central bore therethrough, the annulus being formed between a first casing string and a second casing secured within the wellhead assembly and extending into the well, the method comprising:
providing a port in the wellhead assembly having an opening into the pressure boundary region of the central bore;
securing the first casing string at a first end in the wellhead assembly below the opening of the port in the central bore; and
securing the second casing string at a first end in the wellhead assembly above the opening of the port in the central bore;
thereby forming a fluid flowpath comprising the port in the wellhead assembly and the annulus between the first and second casing strings.
The method may advantageously be employed to pass a fluid into the wellhead assembly and the well, in particular a slurry of cuttings for injection into an underground formation. As an alternative, the method may be employed to produce fluids from the well through the annulus, such as drilling muds.
As noted above, it is preferred to provide the flowpath with a deviation in direction between the annulus formed between the first and second casings and the port in the wellhead assembly. In this way, erosion of the casings and casing hangers is reduced. As also noted above, it is preferred if the flowpath comprises at least two deviations in direction between the aforesaid points in the wellhead assembly.
It is an important advantage of the method of this invention that the injection of fluids into the well or the production of fluids from the well may take place while other well and downhole operations, such as drilling, are underway, without interruption of the latter.
Specific embodiments of the apparatus and method of the present invention will now be described in detail having reference to the accompanying drawings. The detailed description of these embodiments and the referenced drawings are by way of example only and are not intended to limit the scope of the present invention.